Generally, a derailleur for a bicycle has a parallelogram linkage mechanism comprising a fixing member, two parallel link members supported thereto, and a movable member supported to the link members. The linkage mechanism is transformed to move the movable member, so that a change-over frame supported thereto and having two pulleys switches a driving chain to a selected one of the multi-stage sprockets.
In this derailleur construction, since the two pulleys, i.e., the tension pulley and the guide pulley, move following movement of the movable member, the drive chain, which travels from a front chain gear, enters the derailleur in different positions every time for changing speed, thereby causing the drive chain to rock. When such a derailleur is applied to a bicycle for a motorcross race, the rocking is particularly significant causing the chain to come off the sprocket.
This type of bicycle derailleur, employing the linkage mechanism, is also complicated in construction, especially, the two-stage speed change type used for a motocross race bicycle, and is expensive to produce.